The son of a Kenyan government official has been identified as one of the gunmen who took part in an Islamic extremist attack on a university that killed 148 people.

Interior Ministry spokesman Mwenda Njoka said the gunman was identified as Kenyan Abdirahim Mohammed Abdullahi. He said Abdullahi's father, a chief in Mandera County, had reported his son missing last year and said he feared that he had gone to Somalia.

Somalia's al-Shabab Islamic militants claimed responsibility for the attack on Garissa university on Thursday saying it is retribution for Kenya deploying troops to Somalia to fight the extremist rebels and vowed to carry out further attacks.

Kenyan President Uhuru Kenyatta has declared three days of mourning and pledged to take harsh measures against the Islamic militants.

In a nationally televised address, Mr Kenyatta said his administration "shall respond in the severest ways possible" to the Garissa attack, which occurred when four gunmen entered a campus and slaughtered students. The military moved in hours later and the gunmen were killed.

"We will fight terrorism to the end," said Mr Kenyatta. "I guarantee that my administration shall respond in the fiercest way possible."

Mr Kenyatta said the country's "security forces are pursuing the remaining accomplices. We will bring all of them to justice ... We are also in active pursuit of the mastermind (of the Garissa attack) and have placed a reward for his capture."

Five people have been arrested on suspicion of involvement in the Garissa attack, a Kenyan official said.

Meanwhile grieving Christians prayed, sang and clapped hands at an Easter Sunday service at a Catholic church in Garissa.

Security forces patrolled the perimeter of Our Lady of Consolation Church, which was attacked by militants almost three years ago. Grenades lobbed at the building sprayed shrapnel into the interior, injuring some worshippers. Another Garissa church was also attacked that day, leaving 17 people dead,

Today's ceremony was laden with emotion for the several hundred members of Garissa's Christian minority, which is fearful following the attack by al-Shabab. The gunmen who attacked Garissa university on Thursday singled out Christians for killing, though al-Shabab has a long record of killing Muslims over the years.

"Thank you for coming, so many of you," Bishop Joseph Alessandro said to the congregation. He said some of those who died in the university attack would have been at the service, and he read condolence messages from around the world.

Bishop Alessandro saw a parallel between the ordeal of Jesus Christ, which Easter commemorates, and that of Garissa.

"We join the sufferings of the relatives and the victims with the sufferings of Jesus," he said. "The victims will rise again with Christ."

Bishop Alessandro first came to Garissa in 1989 and was shot and injured by bandits on a trip outside the town several years later. He said there had been development in recent years in the area, as well as an increase in insecurity because of al-Shabab.

"You don't know who they are. They could be your neighbours," he said. "A heavy security presence only helps up to a point and more intelligence on the militants is needed," he said.

Later Mr Njoka said Abdullahi graduated from the University of Nairobi with a law degree in 2013 and was viewed as a "brilliant upcoming lawyer".

It is not clear where he worked before he disappeared last year, Mr Njoka said.

To prevent an escalation of Islamic radicalisation in Kenya, it is important that parents inform authorities if their children go missing or show tendencies of following violent extremism, said Mr Njoka.